1. Field of the Invention
The present invention relates to an antenna device, having improved wideband frequency characteristics, suitable for being used in a communication system requiring wideband characteristics, such as ultrawideband (UWB) communications where impulses are directly used without carriers in an electrical manner.
2. Description of the Related Art
Conventionally, there has been known a technique for widening a band in wireless communications using carriers, which is shown in FIG. 8.
Referring to FIG. 8, an antenna device according to the related art is configured such that two plate-shaped radiating conductors 53 and 54 are provided on a surface of a plate-shaped dielectric 52 provided to stand on a ground plane 51, the two plate-shaped radiating conductors 53 and 54 being vertically provided parallel to each other and having slightly different lengths from each other, one end of the radiating conductor 53 facing the ground plane 51 is connected to a feed line 55 such as a coaxial cable, one end of the radiating conductor 54 is connected to the ground plane 51, and the radiating conductors 53 and 54 are coupled through a capacitor 56 that adjusts an impedance.
Further, when carrier signals are fed through the feed line 55, the two radiating conductors 53 and 54 resonate at different frequencies so as to radiate electric waves, respectively, because the radiating conductors 53 and 54 are coupled to each other and have slightly different lengths from each other even though the signals having the same frequency are fed thereto. Thus, as shown by a dotted line in FIG. 7, it is possible to widen the band (for example, see JP-A-2003-133838).
However, in a case in which the lengths of the radiating conductors 53 and 54 are greatly different in order to widen the band even more in the conventional antenna device, the radiating conductors 53 and 54 are not properly coupled to each other, and the resonance of the radiating conductor 54 becomes weak because the wavelength of the carrier signal fed thereto and the length of the radiating conductor 54 are greatly different from each other, which makes it impossible to widen the band in a bilaterally symmetrical manner.
Further, when the length of the radiating conductor 54 is set to be extremely different from the wavelength of the carrier signal fed thereto in order to widen the band, the radiating conductor 54 cannot resonate.
Therefore, even though various design conditions, such as a coupling condition, are optimized in the antenna device according to the related art, only several percent of a band can be widened as compared with the band which can be widened when only one radiating conductor is used.
Furthermore, even though conditions, such as the length difference between two radiating conductors, the coupling condition due to the arrangement gap, or the capacitance of a capacitor that adjusts the impedance, are set in the antenna device according to the related art, it is not possible to perform the radiation in a frequency band required for the ultrawideband communications.